The present invention relates to a control apparatus of an internal combustion engine, and more particularly, to a control apparatus capable of satisfactory air-fuel ratio control for fuels of any types having different properties.
In some cases, the properties of fuels used in internal combustion engines may vary in different localities or nationalities and depending on the season. In those countries in which leaded gasoline is being replaced by lead-free gasoline, there is a demand for the use of various fuels with different octane numbers. In engines matched to the use of low-octane fuels, the ignition timing is adjusted so as to tend to delay. If a high-octane fuel is used in these engines, therefore, it cannot fulfill its intrinsic potential for the engine output. In engines matched to the use of high-octane fuels, on the other hand, the ignition timing is adjusted so as to tend to advance. If a low-octane fuel is used in these engines, therefore, the engines will possibly knock and be damaged.
In order to avoid these awkward situations, an arrangement for ignition timing control (knock control) for the fuel used has been conventionally employed wherein an octane-number selector switch is provided which is shiftable according to the octane number of the fuel used. By shifting this switch, a basic ignition timing map is selected corresponding to the octane number of the fuel used. Also, a knock sensor is attached to the engine, whereby a retard amount is determined in accordance with a knock amount. Thus, a basic ignition timing calculated on the basis of a value read from the basic ignition timing map in accordance with the retard amount is corrected, so as to be suited to the fuel used.
When a low-octane fuel is used, however, exhaust gas cannot be prevented from increasing its temperature so much that the components of an exhaust system are damaged, by only setting the ignition timing so as to tend to delay. To eliminate this disadvantage, the octane-number selector switch shiftable according to the octane number of the fuel used can be used also for the selection of a basic injection quantity map. In this case, the basic injection quantity map is selected corresponding to the octane number of the fuel used, so that a basic injection quantity is adjusted to values corresponding to the rich and lean sides for low- and high-octane fuels, respectively.
However, if the octane-number selector switch is manually operated to select the basic ignition timing map and the basic injection quantity map, it is liable to suffer wrong operation or miss being operated. For example, if the selector switch is shifted to the low-octane side, under a misapprehension that a low-octane fuel is being used, the ignition timing is gradually switched to the advance (lead) side by knock control. Finally, the ignition timing is advanced to a point of time just before knocking is detected by the knock sensor. In this case, the air-fuel ratio is set on the rich side for the supposed use of the low-octane fuel, so that the engine cannot be damaged. In contrast with this, if the octane-number selector switch is shifted to the high-octane side, under a misapprehension that a high-octane fuel is being used, the ignition timing is gradually switched to the delay side by knock control. Finally, the ignition timing is delayed to a point of time when knocking cannot be detected by the knock sensor. In this ccase, the air-fuel ratio is set on the lean side for the supposed use of the high-octane fuel, so that the exhaust gas may increase its temperature, thereby damagingn the components of the exhaust system. If high-speed, high-load operation is continued, in particular, the possibility of such damaging is high.